Generally in an engine, a large thermal load is applied to a piston, therefore in order to prevent abnormal combustion, such as engine knock due to high temperature of a piston head, a cooling device, which prevents erosion and abnormal combustion of the piston head by ejecting cooling oil to the rear side of the piston, is used.
As depicted in FIG. 9, which is a schematic of a main section of general piston cooling, an oil pump 5, driven by the driving force of an engine, draws up oil from an oil pan (not illustrated) of the engine while the engine is in operation, and an oil cooler 4 cools the oil by cooling water of the engine.
The oil cooled by the oil cooler 4 is injected from an oil injection nozzle 8 to a rear face of a piston 1, whereby the piston 1 is cooled.
Japanese Patent Application Laid-Open No. 2006-29127 (Patent Document 1) discloses a cooling device for a piston.
In particular Patent Document 1 discloses a technology comprising: a double structure cleaning channel constituted by a first oil passage (inside) and a second oil passage (outside) formed in a piston head unit 1a; a warm-up oil supply unit which supplies warm-up oil to one of the first oil passage and the second oil passage when cooling the engine; and the warm-up oil supply unit that supplies cooling oil to the other one of the first oil passage and the second oil passage when the piston temperature is high.
Patent Document 1: Japanese Patent Application Laid-Open No. 2006-29127
The oil pump 5, however, is connected to a crankshaft (not illustrated) of the engine via a gear train, hence the oil pump 5 operates simultaneously when the crankshaft of the engine rotates.
Therefore when the engine starts, the oil pump is driven and the oil in an oil pan in a cooled state is injected to the rear face of the piston, whereby the piston is kept cool.
This means that the temperature of the piston head does not rise quickly, and that it takes time until the engine reaches the best operating conditions, in other words startability is not good and fuel consumption is high.
Furthermore according to Patent Document 1, the warm-up oil supply unit that supplies a warm-up oil when cooing the engine and a heating up unit for heating oil are included, which increase the cost of the device, and is also not desirable in terms of fuel consumption.